1. Field of the Invention
The present invention relates to a device for tensioning a rod before locking it, for example with the aid of a nut, particularly for prestressing threaded rods used for fastening mechanical parts to one another.
2. Description of the Relevant Art
Fastening rods can be tensioned by applying a clamping torque to nuts bearing axially on the parts to be assembled. Because of the friction at the face of the nut in contact with an assembled part and at the mating nut/rod threads, this clamping method does not make it possible to control with precision the axial prestressing force on the rod. Furthermore, this clamping method introduces residual torsional stresses in the threaded rod, which leads to the precautionary measure of using oversized threaded rods. Moreover, such a clamping operation leads most often to a relatively high degree of damage not only to the threads of the nut and to those of the threaded rod cooperating therewith while being tightened but also to the surface of the part against which the nut will bear as it is being turned during the clamping operation.
To avoid these disadvantages, it is possible to axially tension a threaded rod before tightening down a clamping nut onto a bearing surface. The clamping nut locks the rod when a tensioning force is released from the rod. The rod is prestressed only along its axis.
In order to exert tension on the rod, use may be made of an actuator or tensioner. Thus, after fitting the nut for clamping the assembly to the end of the rod to be prestressed, the end of the rod together with the nut is covered with the actuator. A puller in the form of a nut in the form of a draw nut is then screwed on to apply a tensile force developed by the actuator. Finally, while the rod is held in tension, the nut of the assembly is tightened down so as to bring it into contact with the part to be assembled, and the tension exerted by the actuator on the rod is released, this rod being locked by the nut of the assembly.
Often, mechanical assemblies must be held together by a number of threaded rods. The use of such pulling devices, especially when the number of threaded rods is high, entails multiple screwing and unscrewing operations for the pullers, making the assembly operations tiresome. Furthermore, the conditions under which the pulling devices are used may be difficult, which makes their use by an operator more testing still.
Document GB 2 201 745 discloses a device for tensioning a threaded rod comprising a pair of radially movable jaws arranged in a conical sleeve. A clasp nut is provided for locking the rod by bearing against a plate. A piston-cylinder assembly is arranged axially between the sleeve and the clasp nut. The piston is designed to push the sleeve axially on the side remote from the nut. An axial movement of the conical sleeve brings the jaws together radially so that they come into engagement with the threaded rod.
This device makes it necessary for the threaded rod to be formed with a free end of considerable length, corresponding to the length of the clasp nut incremented by the length of the piston-cylinder assembly and the jaws, so as to allow the jaws to come into engagement with the threaded rod.
Moreover, the jaws come into engagement with the threaded rod with a combined radial and axial movement during the application of an axial tensioning force. There is a risk that the crests of the threads of the threaded rod will bear against the crests of the threads of the jaws and become damaged.
In addition, the jaws come into engagement with the threaded rod only when an axial tensioning force is applied. Poor cooperation between the jaws and the threaded rod will not be detected prior to the application of an axial force by the piston. An operator must be able to be assured that each tensioning device is properly installed and that a puller will be able to transmit the forces satisfactorily, prior to tensioning a set of rods.
In certain devices, elements are fastened together with the aid of a large number of threaded rods and nuts. The tensioning devices are installed by operators, sometimes under difficult conditions which deprive the operators of total freedom of movement. It is necessary to facilitate the installation of each tensioning device.